If you want to continually monitor I'd use the driver mentioned above (I may be a little biased), but if you only need an occasional quick check:
http://yourHubIp/hub/advanced/freeOSMemory
will give you the value. Right after boot the free memory is usually in the mid 500K range, but will usually drop into the 400K range within 30-40 minutes as the hub gets all of the services and apps running and then there is generally a slow bleed off after that. Depending on your app/driver mix most hubs will run in the 400-300K range for many days/weeks. I have my alerts set at 250K, but usually reboot around 290-280K.
